Carotid arteries supply the head and neck with blood. There is a right common carotid artery and a left common carotid artery. Both the right and left common carotid arteries divide into an external carotid artery and an internal carotid artery. The external common carotid arteries supply blood to the face, scalp, and most of the neck and throat tissue. The internal common carotids supply blood to the brain and other tissue generally accessible from within the scalp, as the eyes. The carotid arteries may suffer from a condition known as stenosis.
Carotid stenosis is an abnormal condition characterized by the constriction or narrowing of these vital arteries by a substance known as plaque, that prevents proper circulation of the blood to the head. Plaque is a localized area of atherosclerosis, that deposits of lipids (fatty substances) and a proliferation of fibrous connective tissue on the inner walls of the arteries. Carotid stenosis may sometimes be treated by a procedure known as endarterechtomy, that is the surgical removal of the innermost layer of the carotid arteries. Endarterechtomy is performed to clear a major artery that may be blocked by the accumulation plaque. In scientific and routine medical work, the measurement of carotid artery stenosis (the measurement of how much the carotid arteries have been constricted due to plaque deposits) has been used to determine whether a patient is likely to benefit from endarterechtomy. High-risk patients may be identified by finding "severe" lesions (a pathological change in body tissue) of the carotid arteries using a very strict method of measurement of stenosis. An article by Allan J. Fox, "How to Measure Carotid Stenosis," Radiology, 186:316-318 (1993) teaches how to identify high risk patients based on the measurement of the extent of stenosis of the carotid arteries (the article is hereby incorporated by reference).
An Angiography-based diagnosis is current standard for determining the extent of carotid stenosis. Angiography is a special X-ray procedure that takes pictures ("angiograms") of blood vessels. This diagnostic technique makes use of a radiopaque contrast medium, which is a chemical substance that does not permit the passage of X-rays or other radiant energy. Angiography is the X-ray visualization of the internal anatomy of the heart and/or blood vessels after the introduction of a radiopaque contrast medium into the blood. The contrast medium may be injected into an artery or a vein or introduced into a catheter (a hollow tube) inserted in a peripheral artery. The catheter is a small, flexible, hollow tube about the size of a thin strand of spaghetti. The radiologist carefully threads it into the blood vessel and guides it to the area to be studied. The radiologist watches the catheter moving through the blood vessels on a special X-ray television screen. When the catheter reaches the site under investigation, X-ray dye is injected through the catheter. An X-ray image of the artery shows the irregularities or blockages.
The term "angiogram" refers to the radiographic image of a blood vessel produced by angiography. Angiograms have darkened areas that represents open channels in blood vessels. These darkened areas are caused by the contrast medium blocking the passage of X-rays. The main problem with using angiography to measure the extent of carotid stenosis is that current methods misuse angiograms. Diagnoses of carotid stenosis using angiograms are currently based on a single two-dimensional X-ray. Using current techniques, although up to four X-ray projections are usually taken from different angles using current techniques, each is only a two-dimensional image. The four selected angiograms provide limited three dimensional information about carotid stenosis. When multiple projections are used, the one which provides the greatest percentage of stenosis is usually adopted. Such methods may significantly underestimate stenosis extent because a single two-dimensional image of an angiographic silhouette cannot accurately depict complex luminal shapes (shapes relating to the tubular cavity of the arteries). An article by Topol and Nissen demonstrates the problems of using angiographic silhouettes to determine complex luminal shapes. See E. J. Topol and S. E. Nissen. "Our preoccupation with coronary luminalogy: The dissociation between clinical and angiographic findings in ischernic hear disease." Circulation, 92(8):2333-2342, October 1995 (this material is hereby incorporated by reference).
The NASCET (North American Symptomatic Carotid Endarterectomy Trial) and the ECST (European Carotid Surgery Trial) are the most known methods to measure carotid stenosis. See FIG. 1. To fully understand the NASCET and ECST methods, see Peter M. Rothwell, Rod J. Gibson, el al, "Equivalence of Measurement of Carotid Stenosis: A Comparison of Three Methods on 1001 Angiograms," Stroke (American Heart Association, Inc.), 25(12):2435-2439, 1994 (this material is hereby incorporated by reference). The NASCET and ECST methods both indicate the degree of stenosis as a percentage reduction in vessel diameter. The minimum diameter of the arteries caused by stenosis (which is the maximum point of blood constriction) (D.sub.minimum) is compared to another diameter that represents the normal diameter of the carotid arteries when the patient is healthy (D.sub.reference), to obtain the change in diameter of the vessel as a percentage (% D). The following general formula is used by both the NASCET and ECST methods: ##EQU1##
D.sub.minimum, the minimum diameter of the carotid arteries, represents the "residual lumen." (A "lumen" is the interior space within the artery.) A residual lumen is the part of the channel in the carotid arteries at the point of a lesion that remains open after the onset of carotid stenosis.
The reference point (D.sub.reference) differs for the NASCET and ECST methods. In the NASCET formula, as seen in FIG. 1, the reference point diameter (D.sub.reference) is taken along a point of the internal carotid artery in a healthy area well beyond an area of the bulb that was caused by stenosis (D.sub.internal carotid beyond bulb). The cross-sectional diameter beyond the bulb caused by stenosis is assumed to be approximately the diameter of the carotid arteries at the point of stenosis prior to the onset of stenosis. In the ECST formula, as seen in FIG. 1, the reference point diameter (D.sub.reference) is the estimated normal lumen diameter at the site of the lesion (D.sub.estimated), based on a visual impression of where the normal artery wall was before development of the stenosis. FIG. 1 illustrates these formulas. ##EQU2##
Both the NASCET and ECST methods are highly inaccurate because they try to measure a 3D geometric property based on one two-dimensional view of the artery silhouette. The results of both NASCET and ECST formulas have no physical meaning because a narrowing of the artery section cannot be measured by simply observing the reduction in diameter of the artery without a 3D analysis. Still, one can estimate the degree of stenosis by analyzing the cross-sectional area reduction of the artery at the point of the lesion where the artery walls are narrowed. While analyzing the cross-sectional area of reduction of the artery to estimate the extent of stenosis is useful, it is less accurate than a full-scale three-dimensional analysis.
In order to determine the reduction of the cross-section of the arteries, one can assume circular cross-section lesions and calculate the stenosis degree as the percentage of area reduction. For more information on assuming a circular cross-sectional lesion to determine the degree of stenosis, see, D. Delaere, C. Smets, P. Suetens, and G. Maxchal, "Knowledge-based system for the three-dimensional reconstruction of blood vessels from two angiographic projections," Med. and Biol. Eng. and Comput., November 1991 (this material is hereby incorporated by reference). The formula to calculate the degree of stenosis as the percentage of area reduction of the carotid arteries (assuming circular cross-sectional lesions) is: ##EQU3##
where % A is the percent area reduction of the carotid arteries, D.sub.minimum represents the point of maximum carotid stenosis, and D.sub.reference is the diameter of the carotid arteries at a given reference point.